Apparatus and method for removing the effect of interference magnetic fields in a magnetic transducer device for detecting information magnetically coded on a carrier

ABSTRACT

A magnetic transducer and method of operation is disclosed for removing the effect of interference magnetic fields present when detecting information magnetically coded on a carrier. The transducer is provided with a magnetoresistive compensating element disposed to develop a voltage in response to the interference magnetic fields equal to that developed across a magnetoresistive detecting element; however, the compensating element is positioned out of the magnetic leakage field of the coded information.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic transducer device fordetecting coded magnetic information.

It is applicable in particular to devices for reading coded magneticinformation such as check or card reading devices and magnetic tapeunits, etc.; however, for the sake of simplicity the invention will bedescribed as it applied to a check reading device, with theunderstanding that the description is equally applicable to any otherdevice for reading coded magnetic information.

2. Description of the Prior Art

It is known that present day data processing systems often include datainfeed arrangements which employ carriers or slips bearing codedinformation. These carriers may take various forms, such as, forexample, bank checks, post office checks, deposit or withdrawal slips,identification cards or credit cards, etc. In case of credit cards, thisinformation frequently is contained in a strip across the front or backface of the card.

The information generally consists of a sequence of alpha-numericcharacters printed on the slips, that is a sequence of letters of thealphabet, figures, punctuation marks, etc. which indicate, in the casewhere the slip is a check, for example, the number of the check or theaccount number of its drawer. Each character is formed by a set of barscomposed of magnetic ink deposited on an insulating substrate. Thenumber of bars, the distance between the bars, and their relativedisposition are individual to each character and conform to known codessuch as the CMC 7 code, for example. These bars are magnetized beforebeing processed in the code reader.

Consideration will be given to a check and the corresponding data infeeddevice which is called a "check reader" as an illustrative example of anapplication to which the invention may be adapted. The check readerconverts the coded magnetic information represented by the charactersprinted on the check into a succession of electrical signals. Theseelectrical signals are translated to electronic shaping circuits whichconvert this series of electrical signals into a series of square-waveelectrical pulses which are in turn transmitted to electronic circuitsfor recognizing the characters printed on the check. As soon as thepulses which correspond to this series of square-wave electrical pulses,which in turn correspond to the printed characters, have been decoded,that is to say the characters have been identified, it is possible tocause a calculating unit in the data processing system to which thecheck reader belongs to perform operations relating to the check such asdebiting, crediting, updating the account of its drawer, etc.

In order that the object of the invention may be better understood, thefollowing facts about magnetism should be noted:

To magnetize a magnetic material, the material is first subjected to apositive magnetic field whose strength is sufficient to saturate thematerial, that is to say, for the magnetic induction in the material toreach a limiting value B_(s) as soon as the strength of the magneticfield reaches a certain value H_(s). The magnetic field is then removed.There then remains a magnetic induction termed the residual induction (+Mr) which is other than zero and which is characteristic of thematerial. In other words, magnetizing a magnetic material amounts tosaturating it magnetically.

A magnetic material which has been magnetized sets up a magnetic leakagefield H in the immediate vicinity of its surface, and the magnetic fluxof a magnetic field H through an area S is equal to the product of thestrength of this field multiplied by the size of the area.

Generally, check readers comprise a magnetizing device and a magnetictransducer device. The magnetizing device is used to magnetize the barsforming the characters printed on the check in order to render the valueand sense of the magnetic induction identical in all the bars since thisis necessary because when the characters are printed on the check,either the induction in the bars is zero or else the value and sense ofthe magnetic induction varies from one bar to the next throughout thebars. Thus, the magnetic induction in the bars is equal to the residualinduction of the magnetic ink when they are no longer subject to themagnetic field of the magnetizing device.

The magnetic transducer device is sensitive to the magnetic leakagefield which is set up by the bars after they have been magnetized by themagnetizing device and emits an electrical signal in response to thismagnetic leakage field. This signal is transmitted to the aforementionedelectronic shaping circuits. In other words, it can be said that themagnetic transducer device enables the presence of the bars to bedetected.

The check is moved by a mechanical transporting device and is positionedin the device in such a way that all the bars pass in front of themagnetizing device and in front of the magnetic transducer device insuccession in close proximity thereto. The mechanical check transporterdevice may be operated either manually or by an electronic motor.

In present day practice, magnetic transducer devices are formed by ahead consisting of a magnetic circuit which is provided with a wide airgap and around which is coiled a winding containing a large number ofturns. The bars pass by the air gap at a very short distance from it sothat it picks up the major proportion of the magnetic flux which isgenerated by the magnetic leakage field of the bars, via the magneticcircuit of the head. An electrical signal is then received at theterminals of the winding whose voltage is equal in absolute value to thevariation, per unit of time, of the magnetic flux picked up by themagnetic circuit. It can be shown that this voltage is proportional tothe speed of movement of the bars past the air gap. It follows that thisvoltage is sensitive on the one hand to the speed of movement and on theother hand to variations in it (for example, in the case of a checktransporter device which is manually operated), which makes forinaccurate detection of the bars. Such a magnetic head needs care inmanufacture and is relatively expensive and bulky.

Magnetic transducer devices having at least one magnetoresistor overcomethese disadvantages. Magnetoresistors are electrical resistors which aredeposited on a substrate of insulating material in the form of thinlayers or films of very shallow depth (a few hundred Angstroms to a fewmicrons thick) and whose resistance varies when they are subjected tothe flux of a magnetic field. A measuring magnetoresistor R of this kindis connected to the terminals of a generator which outputs a current I.When a bar passes in front of the magnetoresistor of the flux of themagnetic leakage field H causes a change ΔR in its resistance and thus achange ΔV=IΔR in voltage, which means that ΔV/V=ΔR/R, ΔR/R being termedthe coefficient of a magnetoresistance. This coefficient is generally afew percent and is very often negative.

The corresponding electrical signal is amplified and transmitted to theaforementioned shaping circuits. The signal is unaffected by the speedof movement of the bars.

In present day practice, magnetoresistive magnetic transducer devicesinclude, for preference, two magnetoresistors for detecting the presenceof bars which are deposited on a single insulating substrate and whichare positioned a short distance d apart, the bars passing in front ofeach of the magnetoresistors in succession.

The distance d depends in particular on the width of the bars and themaximum and minimum allowable spacing between them. Such a device isdescribed, for example, in an article by G. E. Moore, Jr. and Lijcoteentitled "Dual Strips Magnetoresistive Read Heads for Speed-InsensitiveTape Readers" which was published in the journal "IEEE Transactions ofMagnetics", Vol. 12, Number 6 of November 1976.

Such magnetoresistive devices have the drawback of being extremelysensitive to magnetic and electromagnetic fields other than the magneticleakage field H of the bars. Such fields will be referred to generallyin what follows as magnetic interference fields. Among them may bementioned the magnetic fields which are set up by electrical apparatusof all kinds and the magnetic field of the earth.

Even if these fields are weak and are less than the magnetic leakagefield H of the bars, their effect on the magnetoresistors for detectingthe presence of the bars is nevertheless to disturb the electricaloutput signal from the resistors due to the field H by generating by nomeans negligible electrical signals termed "noise" signals. In otherwords, the overall electrical output signal from the magnetoresistors isformed by the electrical "noise" signals due to the interference fields,superimposed on the signal due to the leakage field H of the bars. As aresult there may be a considerable risk of errors occurring in detectingthe presence of the bars.

SUMMARY OF THE INVENTION

The present invention enables this drawback to be overcome by arranginga compensating magnetoresistor such that it is positioned close to themagnetoresistors for detecting the presence of the bars and off the pathfollowed by the bars when they pass in front of the magnetoresistors. Inthis way, the compensating magnetoresistors is subject to the sameinterference magnetic fields as the detecting magnetoresistors. Theoutput signal from the compensating magnetoresistor is subtracted fromthe output signal from each detecting magnetoresistor, and there isobtained a bar detection signal which is associated with each detectingmagnetoresistor and which is unaffected by the interference magneticfields.

In this way there is produced a simple and inexpensive magnetictransducer device and method for detecting coded magnetic informationwhose reliability in detecting the presence of bars is greater than thatof known transducer devices and detection method. It can be shown thatin the extreme case it is possible to manage with only one compensatingmagnetoresistor.

In accordance with the invention, there is provided a magnetictransducer device for detecting coded magnetic information. Thetransducer device includes at least one magnetoresistive element fordetecting the coded information which is arranged on an insulatingsubstrate. The coded magnetic information passes in front of themagnetoresistive element parallel to its surface and a compensatingmagnetoresistive element is arranged, close to the detectingmagnetoresistive element, in such a way that it is not subject to themagnetic leakage field of the coded magnetic information, but is subjectto the same interference magnetic fields as the detecting element. Thevoltages of the electrical output signals emitted by the said detectingand compensating elements in response to the action on them of theinterference magnetic fields being substantially the same and thesesignals may be subtracted from such other to cancel the effects ofinterference magnetic fields and obtain the bar detection signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be apparent from thefollowing description, which is given by way of non-limiting examplewith reference to the accompanying Figures. In the drawings:

FIG. 1 shows the relative disposition of check bearing bars of magneticink and a prior-art magnetoresistive magnetic transducer device fordetecting the bars made up on FIG. 1a and FIG. 1b, FIG. 1a being a sideview and FIG. 1b being a perspective view.

FIG. 2 is a block diagram of a magnetic transducer device according tothe invention having a single magnetoresistor for detecting the presenceof bars.

FIG. 3 is a block diagram of a magnetic transducer device according tothe invention having two magnetoresistors for detecting the presence ofbars.

FIGS. 4 and 5 show a preferred embodiment of the magnetic transducerdevice of which the block diagram appears in FIG. 3, FIG. 4 being athree-quarter perspective view and FIG. 5 being a sectional view fromthe side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 is seen, in cross section, a carrier or slip C such as a bankcheck which is moving along an axis O (from left to right as viewed inFIG. 1a) at a speed V above a check reader LC, the slip beingtransported by a mechanical check transporting device which is, forexample, manually operated. For the sake of simplicity this device hasnot been shown in FIG. 1. The slip C carries bars BA₁, BA₂, BA₃, BA₄ . .. BA_(n) of magnetic ink which forms part of the characters printed onthe slip. The path followed by the characters when they move above thecheck reader LC defines a reading track PL of width l.

The check reader LC includes a magnetizing device DA, represented by arectangle in FIG. 1, mounted on substrate S and which serves tomagnetize the bars before they pass in front of the magnetic transducerdevice DTM for detecting their presence. In present day practice thelatter is preferably formed by magnetoresistive elements MR₁, MR₂ . . .MR_(n) for detecting the presence of the bars BA_(n), n usually beingequal to 2. A device similar to DTM employing two magnetoresistors isdescribed in the abovementioned publication (IEEE Transactions onMagnetics).

The magnetoresistive elements MR₁, MR₂ . . . MR_(n) are deposited on thesame substrate S of electrically insulating material. Their length L isgreater than the width l of the reading track PL. The distance betweenthe elements MR₁, MR₂ . . . MR_(n) is equal to d. The elementspreferably have the same resistance R and identical coefficients ofmagnetoresistance ΔR/R. The ends of the elements are connected to thesame electrical current generator (not shown in FIG. 1) and a current Iflows through them in the direction of their axial length L (see FIG.1b).

As can be seen in FIGS. 1a and 1b, the elements MR₁, MR₂ . . . MR_(n)are subjected to the magnetic leakage field H of the bars, whoseprincipal lines of force each appear as broken lines in FIG. 1a. Thismagnetic leakage field represented by the H arrow in FIG. 1b isperpendicular to the axial length L of the elements MR₁, MR₂ . . .MR_(n) and causes an electrical signal of voltage ΔV to appear at theterminals of each of the elements, with ΔV=IAR.

Consideration will first be given to a magnetic transducer device DTM₁having only a single magnetoresistive detecting element. In accordancewith the invention, and as can be seen in FIG. 2, the magnetoresistivedetecting element 1 has associated with it a compensatingmagnetoresistive element 2 which is arranged close to member 1, but offthe reading track PL. For technical reasons, it is preferable for themembers 1 and 2 to be identical, i.e. to be of the same length L andresistance R and to have the same coefficient of magnetoresistance ΔR/Rand to be deposited on the same substrate, and to connect the members 1and 2 to the terminals of the same electrical current generator (notshown in FIG. 2) so that they carry the same current I.

The magnetoresistive detecting element 1 in the reading track PL issubjected to the magnetic leakage field H of the magnetic ink bars andto the sum Σ₁ H_(p) of the magnetic and electromagnetic interferencefields other than the magnetic leakage field H of the bars which aresituated around it. The compensating magnetoresistive element 2, on theother hand, is subjected only to the sum Σ₂ H_(p) of the magneticinterference fields made up by the fields other than field H which aresituated around it. Virtually, Σ₁ H_(p) =Σ₂ H_(p).

For each bar which passes above the transducer device DTM₁, thereappears at the terminals 3 and 4 of the magnetoresistive element 1 anelectrical signal of voltage ΔV₁ =V_(S) +₁ V_(p), ΔV_(S) being thesignal representing the voltage across terminals 3 and 4 due to themagnetic leakage field H of the bar. Δ₁ V_(p) being the signalrepresenting the voltage across terminals 3 and 4 due to the effect ofthe magnetic interference field ΣH_(p) on the element 1.

At the terminals 5 and 6 of the compensating magnetoresistive element 2appears a voltage Δ₂ V_(p) which is due to the effect of theinterference magnetic fields on this element. Δ₁ V_(p) is practicallyequal to Δ₂ V_(p), normally to within a few percent by virtue of thepositioning of element 2 outside the leakage field of the bars. It isthen only necessary to associate with elements 1 and 2 an electronicdifferential amplifier circuit (not shown), which receives the twovoltages ΔV₁ and Δ₂ V_(p) at its input terminals, to obtain from itsoutput a voltage which is directly proportional to ΔV_(S). In this waythe magnetic transducer device according to the invention virtuallyprevents interference magnetic fields from having any effect on thesignal for detecting the presence of the bars.

FIG. 3 shows another embodiment of the invention which includes twoidentical magnetoresistive elements 11 and 12 and a compensating singlemagnetoresistive element 13. The single element 12 is similar toelements 11 and 12 in its material composition, width, thickness andcoefficient of magnetoresistance, but is only half as long. This meansthat its resistance is only half as great. The resistance of elements 11and 12 is R and their length is L.

The two magnetoresistive elements 11 and 12 are connected in parallel tothe terminals of the same electrical current generator (not shown inFIG. 3) and in series with element 13. Because of this, if element 13carries a current I, elements 11 and 12 carry a current I/2.

When a bar passes in front of elements 11 and 12, they are subjected tothe magnetic field H of the bar and to interference magnetic fieldsΣH_(p), element 13 being subject only to the fields ΣH_(p).

The voltages ΔV_(P1), ΔV_(P2), ΔV_(P3) due to the effect of theinterference fields ΣH_(P) on elements 11 to 13, which appear atterminals 14 and 15 of element 11, terminals 16 and 17 of element 12,and terminals 18 and 19 of element 13, are as follows, if, for example,it is assumed that their coefficient of magnetoresistance is equal to2%:

    ΔV.sub.P1 =ΔV.sub.P2 =2%×R×1/2=R1/100.

    ΔV.sub.P3 =2%×R/2×1=R1/100.

thus: ΔV_(P3) =ΔV_(P1) =ΔV_(P2).

Then, as in the case of transducer device DTM₁, it is merely necessary,for example, to apply the generated voltages ΔV_(P3) and ΔV_(P1) on theone hand, and ΔV_(P3) and ΔV_(P2) on the other hand, to the inputs oftwo differential amplifiers. The two inputs to each differentialamplifier are subtracted to obtain at the outputs of the latter,bar-detection voltages ΔV_(S1) and ΔV_(S2) from which any noise signaldue to the effect of interference magnetic fields has been moved.

It is clear that without thereby exceeding the scope of the invention,the elements 11, 12 and 13 could have different characteristics (such asresistance per unit length, coefficient of magnetoresistance length L,etc.), the important feature being that elements 11, 12 and 13 aresubject to the same magnetic interference fields such that

    ΔV.sub.P3 =ΔV.sub.P1 =ΔV.sub.P2.

always applies.

It is clear that the technical considerations and theory of operation asdescribed for transducer devices DTM₁ and DTM₂ would also apply todevices DTM₃, DTM₄, DTM_(n) having three, four, and n magnetoresistiveelements for detecting the presence of bars. Thus, in the case of adevice DTM₃ having three detecting elements whose resistance is R, whoselength is L and whose coefficient of magnetoresistance is ΔR/R, theresistance of the compensating magnetoresistance element would be R/3and its length L/3 (for a coefficient of magnetoresistance equal toΔR/R).

The production of the device DTM₂ according to the invention will bebetter understood if a description of the process of manufacture isgiven, this being illustrated by FIGS. 4 and 5 and comprising thefollowing steps:

(1) First Step

A first layer 21 of silicon monoxide (chemical formula SiO) is depositedon a substrate 20 of an electrically insulating material which is a goodconductor of heat, such as glass, ceramic or alumina. This produces anelectrically insulating surface which also allows a good adhesion of thelayers which are deposited subsequently on this surface.

(2) Second Step

The magnetoresistive elements 11, 12 and 13 are then deposited on thelayer 21 by a known technique, such as vacuum evaporation. The threeelements are deposited simultaneously so that their geometrical,electrical and magnetic properties will be identical (in particulartheir length L, thickness, resistance R and coefficient ofmagnetoresistance). The material selected to form the elements 11, 12,and 13 is preferably a nickel/iron alloy (18% iron and 82% nickel).Their thickness is approximately 1000 Angstroms. In a preferredembodiment of the invention the length L is 6 mm, the length of element13 being 3 mm. The distance d is equal to 0.5 mm while the distancebetween the terminal 19 of element 13 and the terminals 14 and 16 ofelements 11 and 12 is 1 mm.

(3) Third Step

Two bevels CH₁ and CH₂ are formed on the substrate 20 as shown in FIG.4, the two bevels forming an angle of less than 45° with the plane ofthe upper face of the substrate.

(4) Fourth Step

A conductive layer (of copper, for example) approximately 1 micron thickis deposited on the layer 21 and arranged in such a way as to producethe connections between the various magnetoresistive elements. In thisway, connections 22, 23, 24 and 25 are produced, connection 22 beingconnected to terminal 18 of magnetoresistor 13 and connection 23 beingconnected to terminal 14 of element 11, terminal 19 of element 13 andterminal 16 of element 12, while connection 24 is connected to terminal15 of element 11 and connection 25 is connected to terminal 17 ofelement 12. The connections 22, 24 and 25 extend onto each of the bevelsCH₁ and CH₂ and terminate at respective contacts 26, 27, 28 and 29 eachof which are relatively large in area compared to the depositedconductive layer.

(5) Fifth Step

Flexible wires 30, 31, 32 and 33 are attached, by tin-brazing, forexample, to contacts 26, 27, 28 and 29, respectively. As can be seen inFIG. 5, which is a sectional view of the substrate taken at the pointwhere contacts 26 and 29 are situated, the blobs of solder 34, 35 do notproject above the level of the upper plane P of the substrate 20, toprevent the check C from coming into contact with them when it passesabove the magnetoresistive elements 11 to 13.

(6) Sixth Step

A protective layer 36 (not shown in FIGS. 4 and 5 for the sake ofclarity by symbolized by an arrow) of SiO₂ is deposited on the upperplane P of the substrate. The thickness of this layer is between 1 and30 microns and covers the magnetoresistive elements, conductive stripsand their terminal connections. This layer 36 provides protection forthe magnetoresistive elements 11 to 13 against all kinds of chemical ormechanical attack.

The embodiment of magnetic transducer device according to the inventionwhich has been described has particular application for detecting thepresence of magnetic ink bars making up magnetic characters on check,but it will be clear to those skilled in the art that the invention canbe applied to the detection of coded magnetic information of any kindand in particular magnetic information recorded on magnetic tapes orstrips.

We claim:
 1. A magnetic transducer for detecting informationmagnetically coded on a carrier adapted to be passed in front of thetransducer comprising a substrate, at least one magnetoresistivedetecting element on said substrate having a surface parallel to theplane in which the carrier moves for detecting the information coded onthe carrier; a magnetoresistive compensating element, said compensatingelement being positioned with respect to said detecting element suchthat said compensating element is not subject to the magnetic leakagefield of said coded information as it passes in front of saidtransducer, but is subject to the same interference magnetic fields asthe detecting element and the voltages developed across the detectingand compensating elements in response to the effect on them of theinterference magnetic fields is the same.
 2. A device according to claim1 wherein the magnetoresistive detecting and compensating elements aremade of the same material.
 3. A device according to claim 1 wherein theresistance per unit length of the detecting and compensating elements isthe same.
 4. A device according to claims 1, 2 or 3 wherein thedetecting and compensating elements have the same coefficient ofmagnetoresistance.
 5. A device according to claim 4 wherein themagnetoresistive detecting elements are n in number, n being a wholenumber greater than one.
 6. A device according to claim 5, wherein R andL are the resistance and length respectively of the n detectingelements, and the resistance and length of the compensating element areequal to R/n and L/n respectively.
 7. A device according to claim 6wherein the number of detecting elements is a whole number greater than1 and the resistance and length of the compensating element are lessthan the resistance and length of the reading elements.
 8. A deviceaccording to claim 6 wherein the detecting and compensating elements aredeposited on the same substrate.
 9. A device according to claim 1wherein the detecting and compensating elements are deposited on thesame substrate.
 10. A device according to claim 4 wherein the detectingand compensating elements are deposited on the same substrate.
 11. Amethod for removing the effect of interference magnetic fields whiledetecting information magnetically coded on a carriercomprising:subjecting the coded information to be detected to a magneticleakage field and a magnetic interference field; developing a firstvoltage corresponding to the effect of the magnetic leakage field of thecoded information and the effect of the interference magnetic fields,developing a second voltage corresponding only to the effect of theinterference magnetic fields and deriving from said first and secondvoltage an electrical signal corresponding only to the effect of themagnetic leakage field of the coded information.
 12. A method as setforth in claim 11 wherein said step of deriving an electrical outputsignal corresponding only to the effect of the magnetic leakage fieldcomprises subtracting the second voltage from the first voltage toobtain an output signal unaffected by said interference magnetic fields.13. A method as set forth in claim 11 or 12 wherein the codedinformation is carried on a carrier in a plane parallel to the plane ofthe surface of the magnetic transducer.
 14. A method as set forth inclaim 11 or 12 wherein the information as carried on a carrier in aplane parallel to the plane of the surface of the magnetic transducerand said information is magnetized prior to being passed by saidtransducer.
 15. A method for removing the effect of interferencemagnetic fields while detecting information magnetically coded on acarrier comprising:passing the coded information on the carrier past amagnetic transducer having a defined reading track, subjecting amagnetoresistive detecting element disposed along the track to themagnetic leakage field of the coded information and to interferencemagnetic fields generated external to the coded information fordeveloping a first voltage across said detecting element correspondingto the effect of the magnetic leakage field of the coded information andthe interference magnetic fields, subjecting a magnetoresistivecompensating element disposed in the proximity of the track to theinterference magnetic fields only generated external to the codedinformation for developing a second voltage across said compensatingelement corresponding only to the effect of the interference magneticfields and processing said first and said second voltages to develop anelectrical signal corresponding only to the effect of the magneticleakage field of the coded information.
 16. A method as set forth inclaim 15 wherein said step of processing includes subtracting the secondvoltage developed from the first voltage developed.
 17. A method as setforth in claims 15 or 16 wherein the information as carried on a carrierin a plane parallel to the plane of the surface of the magnetictransducer and said information is magnetized prior to being passed bysaid transducer.
 18. A magnetic transducer for detecting informationmagnetically coded on a carrier adapted to be passed in front of thetransducer, the path followed by the coded information defining areading track PL of width l, comprising a substrate, at least onemagnetoresistive detecting element on said substrate having a surfaceparallel to the plane in which the carrier moves and beneath the trackPL for detecting the information coded on the carrier, amagnetoresistive compensating element on said substrate having a surfaceparallel to the plane in which the carrier moves, said compensatingelement being positioned away from said detecting element and outsidesaid track PL such that said compensating element is not subject to themagnetic leakage field of said coded information as it passes in frontof said transducer, but is subject to the same interference magneticfields as the detecting element and the voltages developed across thedetecting and compensating elements in response to the effect on them ofthe interference magnetic fields is the same.
 19. A device according toclaim 18 wherein the magnetoresistive detecting and compensatingelements are made of the same material such that the resistance per unitlength of the detecting and compensating elements is the same and thedetecting and compensating elements have the same coefficient ofmagnetoresistance.
 20. A magnetic transducer according to claim 19wherein the number of detecting elements are n in number, n being awhole number greater than 1 and wherein R and L are the resistance andlength respectively of the n detecting elements and the resistance andlength of the compensating element are equal to R/n and L/nrespectively.
 21. A magnetic transducer as set forth in claim 20 whereinthe length of the detecting elements is greater than the width of thereading track.
 22. A magnetic transducer device comprising means fordetecting information magnetically coded on a carrier, said means beingexposed to the coded information in the form of a magnetic leakagefield, said means also being exposed to magnetic interference fields,means for developing a first voltage corresponding to the effect of themagnetic leakage field of the coded information and the effect of theinterference magnetic fields, means for developing a second voltagecorresponding only to the effect of the interference magnetic fields andmeans for deriving from said first and said second voltage an electricalsignal corresponding only to the effect of the magnetic leakage field ofthe coded information.
 23. A transducer as set forth in claim 22 whereinsaid means for deriving an electrical output signal corresponding onlyto the effect of the magnetic leakage field comprises a differentialamplifier connected to receive said first and said second voltage andsubtract the second voltage from the first voltage to obtain an outputsignal unaffected by said interference magnetic fields.
 24. A transduceras set forth in claim 23 having a surface above which the carrierpasses, the coded information being carried on the carrier in a planeparallel to the plane of the surface of the magnetic transducer, thepath followed by the coded information defining a reading track PL ofwidth 1, said means for developing the first voltage comprising at leasta set of magnetoresistive detecting elements, each set including atleast a pair of said elements, said means for developing the secondvoltage comprising a single magnetoresistive compensating element foreach set of detecting elements, said sets of detecting elements beingdisposed on said surface below the path of said reading track and eachsaid compensating element being disposed on said surface adjacent theassociated set of detecting elements and off the reading track such thatsaid magnetoresistive detecting elements are subjected to the magneticleakage field of the coded information and to interference magneticfields generated external to the coded information for developing saidfirst voltage and said magnetoresistive compensating element issubjected only to the interference magnetic fields generated external tothe coded information for developing a second voltage.
 25. A magnetictransducer according to claim 24 wherein each set includes n detectingelements, n being a whole number greater than 1 and wherein R and L arethe resistance and length respectively of the n detecting elements andthe resistance and length of the compensating element are equal to R/nand L/n respectively.